Method of controlling high frequency transmitters



Nov. 22, 1938. E. SCHULZE-HERRINGEN 2,137,629

METHOD OF CONTROLLING HIGH FREQUENCY TRANSMITTERS 5 Sheets-Sheet -l- Filed May 2, 1955 K e w w b w H, 0 0 a a L H 6 Z. m Ill 0 f x 3 1% J W H IHMIIII illlli H n {:1 f W M/ L 7 NH \M \J nfl 1W v0 3 5 0 I Nov. 22, 1938. E. SCHULZE-HERRINGEN 2,137,629

r METHOD OF CONTROLLING HIGH FREQUENCY TRANSMITTERS Filed Ma a, 1935 5 Sheets-Sheet 2 y mmm kW Nov. 22, 1938. E. SCHULZE-HERRINGEN 2,137,629

METHOD OF CONTROLLING HIGH FREQUENCY TRANSMITTERS Filed May 2, 1935 5 Sheets-Sheet 3 Nov. 22, 1938. SCHULZVE-HERRINGEN 1 2,137,629

METHOD OF CONTROLLING HIGH FREQUENCY TRANSMITTERS Filed May 2, 1955 5 Sheets-Sheet 4 Nov. 22, 1938. E. SCHULZE-HERRINGEN 2,137,629

METHODOF CONTROLLING HIGH FREQUENCY TRANSMITTERS Filed May 2, 1955 5 Sheets-Sheet 5 J Mam Emma Patented Nov. 22, 1938 UNITED STATES PATENT OFFIE Erich Schulze-Herringen,

Braunschweig, Germany, assignor to C. Lorenz Aktiengesellschaft, Berlin-Tempelhof, Germany, a company Application May 2, 1935, Serial No. 19,469 In Germany May 2, 1934 3 Claims.

The invention has for its object to improve the efficiency of high frequency transmitters, and particularly of radio telephonic transmitters. In this respect it has been proposed to employ a method which is based on the following principle.

From the modulation currents a continuous current is derived that acts to displace the working point in dependency upon the modulation amplituda as may be seen for instance from the British Patent 363,480. In this Way it is attained that the carrier amplitude is essentially just so great as to be completely utilized for the modulation, that is to say, care is taken that there shall be no overmodulation. There are known also several improvements upon this method which have for their object to avoid distortions to occur in receivers operating with non-linear rectification. By this method the eiiioiency is considerably increased. The inven tion however is intended to create a method which shall ensure an increase in efiiciency still greater than is the case with the known method. This problem is becoming more and more important as the energy requirement of a highpower transmitter and all its auxiliary devices is considerable, so that by improving the efficiency the current expenses can be diminished considerably.

The invention will be understood from the following description and be particularly pointed out in the appended claims, reference being had to the accompanying drawings in which Figs. 1 to 6 are diagrams relating to the operation of arrangements constructed according to the invention. Fig. 7 is a diagrammatic representation of an embodiment of the invention, Figs. 8 to 11 are diagrams relating to the operation of a modification of the arrangement shown in Fig. '7, Fig. 12 is a diagrammatic representation of this modification.

In accordance with the invention the anode potential and grid bias are dependent upon the modulation amplitude and are simultaneously controlled. The diiierence over the known method is that the anode potential and grid bias are controlled simultaneously, from which there result the advantages or physical difierences which as explained hereafter require consideration in connection with the two methods. With the aforesaid known method either the anode potential or the grid bias is controlled in dependency upon the modulation amplitude, so that the two voltages are not controlled simultaneously.

The invention is based on the consideration that the efficiency of the valve tubes is very bad with the usual transmitters because the voltage is in general utilized ineificiently, for the greatest modulation amplitudes occur only for a short interval of time. The greatest efliciency is only attained if the tube is utilized completely. Therefore according to the invention the anode potential for the respective modulation amplitude is so selected that the tube is just completely utilized. If the modulation amplitude increases then the anode potential is likewise increased and thus a characteristic curve of higher position is selected, this curve being likewise completely utilized.

It is essential that the operating voltage Us. be greater than the threshold voltage Us. If Ua=1OUS then the voltage utilization With a current utilization adapted for the purpose if the working point is located in the zero point of the characteristic and if h:90%, the efficiency is 1 =72%. This efficiency is derived by means of the well known formula wherein J represents the ratio between the anode alternating current and the anode direct current and h the ratio between the anode alternating Voltage and the anode direct voltage. This formula may be found in vol. II of the book Elektronenrohren by Barkhausen, page 117. In the above formula Ja represents the anode alternating current and Ia the corresponding anode direct current. This high efliciency is only obtained by locating the working point in the zero point of the characteristic and by arranging for a complete utilization of the valve characteristic, as shown in Fig. 1. In this event the transmitter will be on the point of operating in a state of overtension. In the case of modulated high frequency, however, in order to ensure an eiiiciency of 72% the tube cannot be utilized completely with the carrier wave in its state of rest, as in the case of a complete utilization thereof the amplitudes when modulated will vary between zero and double the carrier amplitude. For this reason the carrier while in its state of rest, i. e. during the periods of no modulation, should only be utilized to a degree which corresponds to the middle of the characteristic, as shown in Fig. 2. Hereby however the voltage utilization h decreases by half so as to amount to 45%, and hence also the efficiency is diminished from 72% down to 36%. If now the carrier is modulated, and if it is so for the limiting case (highest modulation amplitude) with m=1, wherein m represents the modulating degree, then there results the representation of Fig. 3. In this case the current utilization remains approximately the same with respect to each point. The voltage utilization however is varied, and it is so between zero and the full value (h=0 to The input of continuous anode current remains the same, but the alternating current power is changed; it will be in the case of m=1 equal to (the at rest value of the power being supposed to be equal to 1). The power output thus increases by half, namely from 1 to 1,5 and the efficiency of the tube from 36% to The utilization is however great at rarely occurring peaks only. On the average it only amounts to values of 10 to 15%. With telephonic transmitters therefore the average efficiency of the terminal tubes is only a little greater than 36%.

As regards different modulation degrees it will be seen from Fig. 4 that only with the complete utilization m=1 the complete voltage utilization h:90% is attained at the greatest peaks, whilst if m=0 this voltage utilization has the uniform value h=45%. For all intermediate values the peak values relating to a definite modulation degree are below h=90%.

In order now to improve the potential utilization the continuous anode potential Ua. is, according to the invention, so varied in the rhythm of the variations of the modulation degree that the peaks of the high frequency amplitudes have continuously a voltage utilization h=90%. In the following consideration the durchgriff of the tube is assumed to be equal to zero. This word durchgriff is a familiar German term meaning the throughgrip of the plate of the tube on the electrons between the grids and the cathode which may be expressed as the reciprocal of the amplification factor, namely, In order to obtain for all modulation degrees a voltage utilization of 90% at the peaks the continuous anode potential must be varied between the values 0.5 and 1 Ua. Then, if m=0, that is if the carrier is in its state of rest, an efficiency 1 =72% (h=90%, i=1.6) is obtained. If m=1 the efliciency then will be =54%. With the novel method the efficiency of the terminal tube assumes values which are between 72% and 5 i% whereas in the case of the customary method with a constant anode potential the efficiency varies between 36% and 54% with increasing modulation. The average efficiency and thus the saving over the normal circuit arrangement depends upon the average variation ratio of the modulation.

In the foregoing the durchgriff is assumed to be equal to zero in theoretical relation in order that the anode reaction may be selected; such is not the case however in practice. If the anode reaction is taken into consideration then by varying the continuous anode potential the following results: A number of characteristic curves, Fig. 5, is obtained with respect to different continuous anode potentials. With the normal telephonic transmitter the working point is in most cases located in the zero point of the characteristic (point I, characteristic A). If the anode potential is made less by half then the characteristic B is worked upon. The working point then is no more located in the zero point II of the characteristic B but is located with respect to this characteristic B in the negative portion, namely in point I. The working point for characteristic B has thus been displaced in the negative direction and by a distance DAUa, where D is the dumbgriff the active alternating control voltage for characteristic B having consequently decreased by the same amount. The carrier current is thus decreased at its peaks from the value a. down to b, as will be seen from Fig. 5. In order now to obtain with variable continuous anode potential the same carrier current as in the case of constant anode potential, this displacement of the working point, caused by the anode reaction, is according to the invention compensated by influencing the grid bias. It must be attained that with each magnitude of the continuous anode potential the working point for the respective characteristic is located in the zero point of this characteristic, as for instance in point II, Fig. 5, if half the continuous anode potential is dealt with. Therefore the continuous gridpotential is so regulated that with respect to the at rest value of the carrier the working point is located the zero point of the characteristic for half the normal continuous anode potential. If the transmitter is now modulated then with increasing anode potential the grid bias is increased towards the negative side, and this is done whenever the anode potential Ua is increased by the value DAUa. 'By this means it is ensured that the carrier value of the high frequency will always be of the same constant magnitude in the antenna, as is the case also with the customary transmitters operating with constant anode potential (Fig. 6).

In Fig. '7 the terminal stage of the high frequency transmitter is designated E and the high frequency source together with the prestages of the transmitter designated St. The modulating device M, which consists of a microphone and an amplifier, acts to control the anode potential of rectifier G as well as the grid bias of the terminal stage. The anode rectifier is fed by an alternating current network N. The continuous anode potential is fed in a well known manner from the rectifier G over a filter circuit S or a high frequency choke H to the terminal stage E. The anode feeding means of the control transmitter St or of the prestages are not shown as they are immaterial to the invention. The modulating device M is arranged to influence on the one hand a grid control device A and on the other hand a rectifier B. By the grid control device A the voltage delivery of the grid controlled rectifier G is influenced in a well known maner by displacing the ignition point. Any of the known arrangements for controlling rectifiers of this kind may be employed here. It is possible also to use instead of the grid controlled rectifier another source of electromotive force, such as a machine, i. e. a source which is regulable. The modulation frequencies are however on the other hand rectified also by the rectifier B and act to influence over a filter circuit K the grid bias of the terminal tube E.

The modulation of the high frequency carrier may be effected in any well-known manner, either in the terminal stage E or in the prestage stage St, the means and method employed for modulation being independent of the essential features of the present invention. Thus "the modulation may be performed, for example, by applying to the grid circuit or anode circuit of the vacuum tube E, or to the grid circuit or anodecircuit of a tube in the prestage St, the modulation voltages from device M in well-known manner.

The power for controlling the anode rectifier G and the grid of the terminal stage is small compared to the power saved. The saving of continuous current energy depends upon the ratio between the variations occurring in the modulation. This ratio is very small and is, as has been shown by recent experiments, approximately 0:02, including a great factor of safety. With this value a and a variation of the anode potential from 0.5 Ua to l Ua a saving in continuous current input of about 40% is attained. Consequently the efficiency of the transmitter tube is increased to an average value of 60% whereas with the customary transmitter it is not much greater than 36%. If for reasons due to modula- 'tion the anode potential should not be decreased to such degree then with the same 0' of the magnitude 0.2 and a voltage variation between 0.6 and l Ua saving in continuouscurrent input of about 33 /3% is effected, ,which corresponds to an average efii'ciencyof the tube of about 54%. The savings possible due to the invention are thus considerable.

The efficiency may still further be improved by combining in accordance with a further step of the invention the method just described with the method mentioned herebefore in the opening statement, i. e. the method by which the working point is displaced in dependency upon the modulation amplitude. The control device must then be such that the working point is displaced, that is to say, the grid voltage control device, whose operation has been explained with reference to Fig. 5, is to be omitted. By this means the carrier becomes smaller in the case of the anode potential decreasing. One could arrange for having a predetermined at rest value of the carrier with respect to a predetermined durchgriff D. Since however in most cases the durchgriff is of a definite value with respect to each such carrier value, an additional grid control of the well known kind may be used here.

It is further proposed by the invention to use the novel method with voice frequency amplifiers operating as so-called B-amplifiers in push-pull arrangement.

The ideal characteristic of such a B-amplifier in push-pull connection has the shape represented in Fig. 8. The slope of this characteristic then is a straight line, as will be seen from Fig. 9. With respect to a half-wave of the control frequency the energy conditions may be considered as follows:

The efficiency with a complete utilization of the characteristic is 11=72%, since h=% and j=1.6, as shown herebefore regarding the control by high frequency. With the alternating grid potential decreasing, the efficiency likewise decreases in linear relation with the continuous modulation current. In a like manner to high frequency amplifiers the voltage utilization in the case of B-amplifiers for voice frequencies may, as regards the small values of the voice frequency control voltage be increased again by varying the continuous anode voltage and the continuous grid voltage in the rhythm of the average amplitude variations. The continuous anode voltage cannot 'must all run parallel with each other.

be varied from zeroup to the full value 1 but this variation can only be accomplished in start ing from acer'tain at rest value, such as 0.2 or 0.4. This at rest value depends upon the used tubes as the characteristic curves worked upon In Fig. 10 an example of amplitude variations is shown which are produced for instance by speech and correspond to the curve of the continuous anode voltage with an at rest value 0.3. The continuous grid voltage is controlled in the same shape and magnitude as is shown in the foregoing with respect to the high frequency amplification. For the improvement of the efliciency the same applies as for the high frequency amplification.

In reality the characteristics of push-pull amplifiers are not so ideal as shown in Fig. 8 but have for instance the shape represented in Fig. 11. Such shapes of the characteristic are able in the case of small control amplitudes to cause'eifective distortions with respect to loudness ratios and sound. In addition, such aB-amplifier is in the case of small control amplitudes very sensitive over variations of the continuous anode voltage and grid voltage, as then the Working point in the lower portion of the characteristic is liable to be easily displaced into the negative part of the characteristic so that small control amplitudes will not be amplified at all. The active alternating grid voltage Ust is formed by the difference Ust=DUa--Ug, where D is the durchgriff of the tube, Ua is the operating plate voltage and Ug is the grid potential. If Ug becomes small then Ust depends very much upon DU; and thus upon the continuous anode voltage Ua.

Due to the proposed method of controlling the continuous anode voltage and grid bias in order to improve the voltage utilization, the displacement of the active working point resulting from a variation of the anode voltage will in the case of small control amplitudes no longer cause a failure of an amplification. This will be seen from the foregoing formula Ust=DUaUg as now a decrease of Ug does not entail a decrease of Ua and thus of DUa. The sensitivity of the B-amplifier connection with respect to anode voltage variations is approximately the same whether the control amplitudes at the grid of the push-pull connection be great or small.

In order now to diminish the distortions of loudness ratios and sound, caused by the curvature of the lower part of the characteristic, according to a further step of the invention the act of controlling the continuous grid voltage in order to realize the DUa compensation in accordance with a non-linear form, is so accomplished as to counteract the curvature of the characteristic. Owing to this mode of regulating the grid bias, each control amplitude of the voice frequency has practically another working point on the curvature of the respective characteristic. This mode of eliminating distortions has the advantage that the elimination can be effected in the B-amplifier itself or that owing to such special circuit arrangement a substantial distortion cannot occur.

The elimination of distortions in such a B- amplifier for voice frequency may in accordance with a still further step of the invention be effected at the grid by a non-linear displacement of the working point without the continuous anode voltage being varied. The grid bias is controlled here within a smaller range since it is not necessary to compensate DUa but to provide for the small and large control amplitudes being as far as possible free of distortions.

This will be understood from Fig. 12. The push-pull tubes are designated R. The voice frequencies are in a well known manner led over the transformer T1 and on the grid side directly to the push-pull tubes and are also conducted through an amplifier V to the push-pull arrangement, the amplifier V belonging to the novel control arrangement by which the grid voltage as well as the anode voltage are influenced. N is a non-linear amplifier which according to the foregoing serves for compensating the curvature of the characteristics. Connected to the output of the non-linear amplifier is a rectifier G1 which through a smoothing means A1, such as a filter circuit, acts to control the grid bias. In the output of V another rectifier G2 is disposed by which the anode voltage is controlled. In the case represented it is intended to derive the anode voltage from the alternating current network by means of a grid control rectifier GL. In this case the grid of GL is controlled by the rectifier G2. The controlled anode voltage is then led over a smoothing link A2 of the anode circuit to the push-pull arrangement.

What is claimed is:

l. The method of controlling a high frequency transmitter employing a vacuum tube terminal stage for delivering a high frequency carrier modulated in accordance: with modulating frequencies, said vacuum tube stake including cathode, grid and anode, means for supplying direct current potential to the anode and means for sup-- plying a biasing potential to the gride, so as to maintain the high frequency carrier constant, said method comprising varying the direct current potential of the anode in accordance with the average amplitude variations of the modulating frequencies, displacing the working point of the said terminal stage by the anode reaction resulting from such varying of the anode potential, and simultaneously varying the biasing potential of the grid in accordance with the average amplitude variations of the modulating frequencies so as to compensate for the displacement of said working point whereby the output of the transmitter will be constant magnitude for varying values of anode voltage.

2. An arrangement for controlling a high frequency transmitter employing a vacuum tube terminal stage for delivering a high frequency carrier modulated in accordance with modulating frequencies, said vacuum tube stage including cathode, grid and anode so as to maintain the high frequency carrier constant, comprising means including a grid controlled rectifier for supplying direct current potential to the anode of the vacuum tube terminal stage, a source of modulating frequencies, a control rectifier for rectifying modulating frequencies from said source, means for impressing the direct current potentials resulting from such rectification upon the grid of the grid control rectifier, and means including a rectifier for rectifying modulating frequencies from said source and impressing the resulting direct current potentials upon the grid of the terminal stage in such manner as to increase the negative potential on said grid when the direct current potential supplied to said anode is increased.

3. An arrangement for controlling a high frequency transmitter employing a vacuum tube terminal stage for delivering a high frequency carrier modulated in accordance with modulating frequencies, said 'vacuum tube stage including cathode, grid and anode so as to maintain the high frequency carrier constant, comprising a source of modulating frequencies, two rectifying arrangements for rectifying modulating frequencies from said source, means for varying the anode potential of the vacuum tube terminal stage in accordance with the varying direct current potentials resulting from such rectification by one of said rectifying arrangements, and means for varying the grid potential of the terminal stage in accordance with the varying direct current potentials produced by the other of said rectifying arrangements in such manner as to increase the negative potential on said grid when the direct current potential supplied to said anode is increased.

ERICI-I SCHULZE-HERRINGEN. 

